This invention relates to a brake force regulator for a motorcycle hydraulic brake system in which a rear-wheel brake and a front-wheel brake are actuated by the pressure from a pedal master cylinder and a further front-wheel brake is actuated by the pressure from a hand master cylinder, and in which the pressure supplied to the rear-wheel brake can be reduced by the brake force regulator dependent on the pressure generated by the hand master cylinder, with the brake force regulator having a stepped piston which is slidable against the force of a control spring and whose larger effective area is adapted to be exposed to the pressure from the pedal master cylinder and whose smaller effective area is adapted to be exposed to the pressure in the rear-wheel brake, and the stepped piston includes a valve which is adapted to be closed upon displacement of the stepped piston against the force of the control spring to interrupt the fluid connection from the pedal master cylinder to the rear-wheel brake.
From German Patent DE-OS No. 2,558,825 a brake force regulator is known which resembles conventional brake force regulators for passenger cars. In this brake force regulator, actuation of the pedal master cylinder initially results in unreduced pressure supply to a front-wheel brake and reduced pressure supply to the rear-wheel brake. If in addition the hand master cylinder is actuated, the pressure thereby generated counteracts the control force of the brake force regulator so that it reduces the pressure of the rear-wheel brake to a greater extent than if only the pedal master cylinder were actuated. In this arrangement, the brake force regulator is so designed that the displacement travel of its stepped piston is sufficient to reduce the pressure in the rear-wheel brake by means of an increase in the pressure of a front-wheel brake.
This prior known brake force regulator has the functional disadvantage that its stepped piston can be displaced also when pressure is built up by the hand master cylinder only, i.e., when only the hand brake, which acts on the front wheel is applied. The displacement of the stepped piston causes pressurized fluid to be drawn from the rear-wheel brake. As a result, a vacuum will be produced in the rear-wheel brake which may draw in air and contaminants that might result in a failure of the brakes.
The subject matter disclosed in the copending U.S. Patent application of H. Oberthuer, Ser. No. 928,927, filed July 28, 1978, now U.S. Pat. No. 4,174,867, issued Nov. 20, 1979, having the same assigned as the present application eliminates this disadvantage. Therein, the pressure generated by the hand master cylinder acts on a piston which also functions as a valve-closure member. Upon actuation of the hand master cylinder, this piston is loaded in the valve-closing direction and the valve seat is subjected to high mechanical loads because it is via the valve-closure member and the valve seat that the entire force necessary for displacement of the stepped piston against the pressure prevailing in the front-wheel brake actuated by the pedal master cylinder is to be transmitted to the stepped piston. In addition, it is not possible in the subject matter of the above-cited copending application to make the cross section of the piston actuated by the hand master cylinder of any size desired. Therefore, for shifting back of the stepped piston a high pressure is necessary which acts at the same time on the second front-wheel brake.